A self-consistent top–down model for differentiation in bimodal suites: application to the Sonju Lake Intrusion–Finland granite system (MN)

The Sonju Lake Intrusion (SLI) is a 1200-meter thick layered mafic intrusion that directly underlies an equally large silicic pluton, the Finland granophyre (FG) within the Beaver Bay Complex of the Mid-Continent Rift (MN, USA). The SLI, with a simple mineralogical and compositional stratigraphy, provides an excellent case study for examining the changes in iron isotope ratios (<i>δ</i>⁵⁶Fe). Here new Fe isotope data along with ⁸⁷Sr/⁸⁶Sr for a set of stratigraphically controlled samples from the SLI and FG are presented. The Fe isotope data show systematic changes within two differentiation sequences found in the lowermost FG as well as the upper portion of the SLI. Specifically, <i>δ</i>⁵⁶Fe is observed to start at low values and increase to heavy values going stratigraphically up through each differentiation sequence. Within the middle portion of the SLI, <i>δ</i>⁵⁶Fe varies between 0 and 0.1. Two samples from the SLI bottom are isotopically lighter than the middle SLI. The origin of the Fe isotope variations is discussed in terms of recently proposed explanations. A quantitative model shows that the observed spatial variation is consistent with the prediction of a temperature gradient model. Using present constraints on equilibrium phase partitioning, the iron isotope variations do not appear consistent with production by fractional crystallization. Based on these observations, a top–down sill emplacement process coupled with <i>in situ</i> differentiation remains a viable alternative model for forming this layered intrusion.